WIP: Add "busfreq" governor to devfreq

So frequencies in devfreq are changed at runtime via governors only. hence, I wrote a simple new governor. But please keep in mind I'm not yet happy with it at all. I push this very early in order to be able to talk about it (and the general idea).

Just a few problems:

• the name: In order to add to the devfreq/busfreq confusion, I named the devfreq governor "busfreq" -.-
• the global data variable. This is ugly. (should we require drivers to "register" with the governor, in order to pass on its state?)
• is there more useful settings than "performance" and "powersave" we can sanely use?
• The first user, the nwl-dsi bridge: Is the call placed at the right location?

It works and can be the right direction to solve this, but it has to improve quite a bit before considering to post upstream and discuss there.